Method and apparatus for stabilizing adjacent bones

ABSTRACT

An apparatus ( 10 ) is provided for implantation into an adjacent pair of vertebral bodies ( 12  and  14 ) having first and second surfaces ( 17  and  19 ) that oppose each other. The apparatus ( 10 ), when implanted, is attached to the adjacent pair of vertebral bodies and stabilizes the vertebral bodies ( 12  and  14 ) while the vertebral bodies fuse together. The apparatus ( 10 ) comprises a platform ( 24 ) having a third surface ( 38 ) extending transverse to the first and second surfaces ( 17  and  19 ). The apparatus ( 10 ) further comprises at least one helical spike ( 50, 52 ) for embedding into each of the adjacent pair of vertebral bodies ( 12  and  14 ) upon rotation of the platform ( 24 ) to attach the helical spike to each of the vertebral bodies and thus fasten the vertebral bodies together. The helical spike ( 50, 52 ) projects from the platform ( 24 ) and extends around a longitudinal axis ( 22 ). The helical spike ( 50, 52 ) has a tip portion ( 58 ) at a distal end ( 62 ) for penetrating the first and second surfaces ( 17  and  19 ) and for screwing into the adjacent pair of vertebral bodies ( 12  and  14 ) as the platform ( 24 ) is rotated. The helical spike ( 50, 52 ) at least partially defines an internal cavity ( 140 ) for receiving material ( 130 ) that promotes fusion of the vertebral bodies ( 12  and  14 ).

This application claims the benefit of provisional application No.60/238,265, filed Oct. 5, 2000.

TECHNICAL FIELD

The present invention is directed to a method and apparatus forstabilizing adjacent bones, and is particularly directed to a method andapparatus for attaching and stabilizing adjacent vertebral bodies whilethe vertebral bodies fuse together.

BACKGROUND OF THE INVENTION

Each adjacent pair of vertebrae in the human spinal column are separatedby an intervertebral disc that makes relative movement of the vertebraepossible. Problems, however, can develop with one or more of the discs,causing severe back pain. In some cases, it is necessary to remove aproblematic disc and to fuse the adjacent vertebrae together in order torelieve pain.

One known method for fusing an adjacent pair of vertebrae followingremoval of a disc is to implant a device, commonly referred to as afusion cage, into the interbody space where the disc was removed. Thefusion cage facilitates fusion of the vertebrae. Typically, proceduressuch as reaming and/or tapping of adjacent vertebrae are required toprepare the adjacent vertebrae to receive the fusion cage. Suchprocedures normally involve substantial cutting of the hard corticalbone of the end plates of the adjacent vertebrae, which can weaken theend plates and lead to collapse of the vertebrae. The fusion cage isthen positioned in the interbody space and into engagement with theadjacent vertebrae. At least one known fusion cage has relativelymovable parts that enable the fusion cage to be expanded after thefusion cage is positioned in the interbody space between adjacentvertebrae. The design of this expandable fusion cage is, however,relatively complex.

Typically, a fusion cage includes an internal cavity that is filled withbone graft material. The fusion cage and the bone graft material promotebone growth that slowly unites the adjacent vertebrae. The typicalfusion cage, while in engagement with the adjacent vertebrae, does notattach to the vertebrae and thus does not resist relative movement ofthe vertebrae, through bending or rotation, along any one of the threeplanes of motion (sagittal, coronal, or horizontal). Rather, the typicalfusion page relies on the viscoelasticity of the surrounding ligamentsto stabilize the adjacent vertebrae.

It is desirable to provide an apparatus for implantation into anadjacent pair of vertebral bodies that attaches to and thus fastens thevertebral bodies while they fuse together despite the forces on theapparatus from human body movement and muscle memory. It is furtherdesirable to provide an apparatus which has a simple one-piececonstruction and which may be implanted into an adjacent pair ofvertebrae without having to prepare the adjacent vertebrae to accept theapparatus by substantial cutting of the cortical bone.

SUMMARY OF THE INVENTION

The present invention is an apparatus for implantation into an adjacentpair of vertebral bodies having first and second surfaces that opposeeach other. The apparatus, when implanted, is attached to the adjacentpair of vertebral bodies and stabilizes the vertebral bodies while thevertebral bodies fuse together. The apparatus comprises a platformhaving a third surface extending transverse to the first and secondsurfaces. The apparatus further comprises at least one helical spike forembedding into each of the adjacent pair of vertebral bodies uponrotation of the platform to attach the at least one helical spike toeach of the vertebral bodies and thus fasten (pin) the vertebral bodiestogether. The at least one helical spike projects from the platform andextends around a longitudinal axis. The at least one helical spike has atip portion at a distal end for penetrating the first and secondsurfaces and for screwing into the adjacent pair of vertebral bodies asthe platform is rotated. The at least one helical spike at leastpartially defines an internal cavity for receiving material thatpromotes fusion of the vertebral bodies.

In accordance with one embodiment of the present invention, theapparatus comprises a pair of helical spikes. The proximal ends of thepair of helical spikes are spaced 180° apart.

In accordance with another embodiment of the present invention, theapparatus comprises three helical spikes extending around thelongitudinal axis. The proximal ends of the three helical spikes arespaced 120° apart.

The present invention also provides a method for attaching andstabilizing an adjacent pair of vertebral bodies while the vertebralbodies fuse together, the vertebral bodies having first and secondsurfaces that oppose each other. The method comprises the step ofremoving disc material disposed between the vertebral bodies to createan interbody space and the step of providing an interbody stabilizer forinsertion into the interbody space by implanting the interbodystabilizer into both of the adjacent pair of vertebral bodies. Theinterbody stabilizer comprises a platform and at least one helicalspike. The platform has a third surface extending transverse to thefirst and second surfaces of the vertebral bodies. The at least onehelical spike projects from the platform and extends around alongitudinal axis. The at least one helical spike at least partiallydefines an internal cavity for receiving material that promotes fusionof the vertebral bodies. The method further comprises the step ofembedding the interbody stabilizer into each of the adjacent pair ofvertebral bodies by rotating the platform of the interbody stabilizer.Rotation of the platform causes the at least one helical spike topenetrate into and subsequently out of each of the vertebral bodies inan alternating manner to attach the interbody stabilizer to each of thevertebral bodies and thus fasten (pin) the vertebral bodies together.Material that promotes fusion of the vertebral bodies is placed into theinternal cavity in the interbody stabilizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic anterior view of an apparatus implanted in anadjacent pair of vertebral bodies in accordance with the presentinvention;

FIG. 2 is a side view taken along line 2—2 in FIG. 1;

FIG. 3 is a perspective view of the apparatus of FIG. 1;

FIG. 4 is a sectional view taken along 4—4 in FIG. 1;

FIG. 5 illustrates an alternate configuration for an end portion of theapparatus of FIG. 1;

FIG. 6 is a schematic anterior view illustrating a second embodiment ofthe present invention;

FIG. 7 is an exploded perspective view of the apparatus of FIG. 6, andincludes a driver for rotating the apparatus;

FIG. 8 is a side view illustrating a third embodiment of the presentinvention;

FIG. 9 is a side view illustrating a fourth embodiment of the presentinvention; and

FIG. 10 is a sectional view taken along line 10—10 in FIG. 9.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to a method and apparatus forstabilizing adjacent bones, and is particularly directed to a method andapparatus for attaching and stabilizing adjacent vertebral bodies whilethe vertebral bodies fuse together. As representative of the presentinvention, FIG. 1 illustrates an apparatus 10 implanted into an adjacentpair of lumbar vertebrae 12 and 14 in a vertebral column (not shown). Itshould be understood that the apparatus 10 could be implanted into anyadjacent pair of vertebrae. The vertebrae 12 has a side surface 16 and alower surface (or end plate) 17 (FIG. 2). The vertebrae 14 has a sidesurface 18 and an upper surface (or end plate) 19.

The apparatus 10 comprises an interbody stabilizer 20 made from abiocompatible material, such as titanium or stainless steel. It iscontemplated that the biocompatible material used to make the interbodystabilizer 20 could also be biodegradable. The interbody stabilizer 20is centered about a longitudinal axis 22 (FIG. 3). The interbodystabilizer 20 includes a platform 24 having a generally cylindricalouter surface 26 extending between oppositely disposed first and secondends 28 and 30. The second end 30 of the platform 24 includes an endsurface 38 that extends transverse to the side surfaces 16 and 18 of theadjacent vertebrae 12 and 14, respectively. The end surface 38 of theplatform 24 has a shape that is complimentary to the side surfaces 16and 18 of the vertebrae 12 and 14, respectively.

The platform 24 of the interbody stabilizer 20 further includes an axialpassage 40 that extends from the first end 28 to the end surface 38. Thepassage 40 has a hexagonal configuration for receiving a rotatabledriver (not shown).

First and second helical spikes 50 and 52 project from the end surface38 of the platform 24. The helical spikes 50 and 52 resemble a pair ofintertwined corkscrews. According to the embodiment illustrated in FIGS.1-4, the first and second helical spikes 50 and 52 extend around theaxis 22. The spikes 50 and 52 extend in a helical pattern about the axis22 at the same, constant radius R1. It is contemplated, however, thatthe first and second helical spikes 50 and 52 could extend about theaxis 22 at different radiuses. Further, it is contemplated that theradius of one or both of the first and second helical spikes 50 and 52could increase or decrease as the helical spikes extend away from theplatform 24. In order for the interbody stabilizer 20 to be implantedendoscopically through a typical cannula (not shown), it is preferredthat the platform 24 and the helical spikes 50 and 52 are less than 20mm in overall diameter. It should be understood that the interbodystabilizer 20 could have an overall diameter that is greater than 20 mmfor certain applications, and that the interbody stabilizer could alsobe implanted in an open surgical procedure. However, for structuralstability reasons, the overall diameter of the helical spikes 50 and 52should remain less than or equal to the diameter of the platform 24.

In the illustrated embodiment of FIGS. 1-4, the first and second helicalspikes 50 and 52 have the same axial length, and also have the samecircular cross-sectional shape. It is contemplated, however, that thefirst and second helical spikes 50 and 52 could have different axiallengths. Further, it is contemplated that the helical spikes 50 and 52could have a different cross-sectional shape, such as an oval shape. Italso contemplated that the first and second helical spikes 50 and 52could have different cross-sectional shapes and/or areas (i.e., onespike being thicker than the other spike). Finally, it is contemplatedthat the helical spikes 50 and 52 should have the same pitch, and thatthe pitch of the helical spikes would be selected based on the specificsurgical application and quality of the bone in which the interbodystabilizer 20 is to be implanted.

Each of the first and second helical spikes 50 and 52 can be dividedinto three portions: a connecting portion 54, an intermediate portion56, and a tip portion 58. The connecting portion 54 of each of thehelical spikes 50 and 52 is located at a proximal end 60 that adjoinsthe end surface 38 of the platform 24. The connecting portion 54 mayinclude barbs (not shown) for resisting pull-out of the helical spikes50 and 52 from the vertebrae 12 and 14. According to one method formanufacturing the interbody stabilizer 20, the connecting portion 54 ofeach of the helical spikes 50 and 52 is fixedly attached to the platform24 by inserting, in a tangential direction, the proximal ends 60 of thehelical spikes into openings (not shown) in the end surface 38 andwelding the connecting portions 54 to the platform. The insertedproximal ends 60 of the helical spikes 50 and 52 help to reduce tensilebending stresses on the helical spikes under a tensile load .

Alternatively, the helical spikes 50 and 52 may be formed integrallywith the platform 24, such as by casting the interbody stabilizer 20. Ifthe interbody stabilizer 20 is cast, it is contemplated that a fillet(not shown) may be added at the junction of the helical spikes 50 and 52and the platform 24 to strengthen the junction and minimize stressconcentrations at the connecting portions 54. The fillet at the junctionof the helical spikes 50 and 52 and the platform 24 also helps to reducebending stresses in the connecting portions 54 of the helical spikesunder a tensile load.

As best seen in FIG. 4, the connecting portions 54 at the proximal ends60 of the first and second helical spikes 50 and 52 are spaced 180°apart about the axis 22 to balance the interbody stabilizer 20 andevenly distribute loads on the helical spikes. The connecting portion 54of each of the helical spikes 50 and 52 has a first cross-sectionaldiameter D1 (FIG. 3).

The tip portion 58 of each of the helical spikes 50 and 52 is located ata distal end 62 of the helical spikes. The intermediate portion 56 ofeach of the helical spikes 50 and 52 extends between the tip portion 58and the connecting portion 54. The intermediate portion 56 and the tipportion 58 of each of the helical spikes 50 and 52 has a secondcross-sectional diameter D2 that is less than or equal to the firstcross-sectional diameter D1 of the connecting portions 54. If the secondcross-sectional diameter D2 is less than the first cross-sectiondiameter D1, the increased thickness of the connecting portions 54 ofthe helical spikes 50 and 52 will help to provide the interbodystabilizer 20 with increased tensile strength at the junction of thehelical spikes and the platform 24.

The tip portion 58 of each of the helical spikes 50 and 52 isself-penetrating and provides the helical spikes with the ability topenetrate into a respective one of the vertebrae 12 and 14 as theplatform 24 of the interbody stabilizer 20 is rotated in a clockwisedirection. The tip portions 58 illustrated in FIGS. 1-4 have anelongated conical shape with a sharp pointed tip 68. FIG. 5 illustratesan alternative, self-tapping configuration for the tip portions 58 whichincludes a planar surface 66 for driving into the vertebrae 12 and 14,in the same manner that a wood chisel turned upside-down drives intowood, as the platform 24 is rotated. It is contemplated that the tipportions 58 could also have a pyramid shape, similar to the tip of anail.

FIGS. 1 and 2 illustrate the interbody stabilizer 20 implanted in theadjacent lumbar vertebrae 12 and 14 to stabilize the vertebrae. First,disk material that normally separates the vertebrae 12 and 14 is removedby the surgeon. Removal of the disk material leaves an interbody space62 (FIG. 2) between the vertebrae 12 and 14. A tool (not shown) is thenused to punch a hole (not shown) in the cortical bone (not shown) ofeach of the vertebrae 12 and 14. The hole in the vertebrae 12 may bepunched in either the side surface 16 or the lower surface 17. The holein the vertebrae 14 may be punched in either the side surface 18 or theupper surface 19. The holes in the vertebrae 12 and 14 are punched inlocations that correspond to the spacing of the tip portions 58 of thehelical spikes 50 and 52 of the interbody stabilizer 20. The holes inthe vertebrae 12 and 14 are intended to make the initial rotation of thestabilizer 20 easier. It should be noted that one or both of theconfigurations of the tip portions 58 illustrated in FIGS. 1-5 may beable to punch through the cortical bone upon rotation of the interbodystabilizer 20, thus eliminating the need for the aforementioned tool topunch holes in the cortical bone.

The tip portions 58 of the interbody stabilizer 20 are placed in theholes in the vertebrae 12 and 14 and a rotatable driver (not shown) isinserted into the passage 40 in the platform 24. The driver is thenrotated, causing the interbody stabilizer 20 to rotate as well. It iscontemplated that a cylindrical sleeve (not shown) may be placed aroundthe intermediate portions 56 and the connecting portions 54 of thehelical spikes 50 and 52 to prevent the helical spikes from deformingradially outward during the initial rotation of the interbody stabilizer20.

Rotation of the interbody stabilizer 20 screws the helical spikes 50 and52 into the vertebrae 12 and 14, respectively. The tangentially-orientedconnection between the connection portions 54 of the helical spikes 50and 52 and the platform 24 minimizes bending loads on the connectingportions during rotation of the interbody stabilizer 20. Further, thetangentially-oriented connection ensures that the force vector resultingfrom axial force torque and applied by the driver to the platform 24 istransmitted along the helical centerline (not shown) of each of thehelical spikes 50 and 52.

As the interbody stabilizer 20 is rotated, the tip portion 58 of thefirst helical spike 50 penetrates the cancellous bone in the vertebrae12 and cuts a first helical segment 82 of a first tunnel 80 (FIG. 1) inthe vertebrae 12. Simultaneously, the tip portion 58 of the secondhelical spike 52 penetrates the cancellous bone of the vertebrae 14 andcuts a first helical segment 102 of a second tunnel 100 in the vertebrae14.

At some point between 90° and 180° of rotation of the interbodystabilizer 20, the tip portions 58 of the helical spikes 50 and 52penetrate back out of the vertebrae 12 and 14, respectively and into theinterbody space 62. More specifically, the tip portion 58 of the firsthelical spike 50 projects through the lower surface 17 of the vertebrae12 and into the interbody space 62. Simultaneously, the tip portion 58of the second helical spike 52 projects through the upper surface 19 ofthe vertebrae 14 and into the interbody space 62.

As the interbody stabilizer 20 is rotated beyond 180°, the tip portions58 of the helical spikes 50 and 52 move through the interbody space 62and engage the vertebrae 14 and 12, respectively. The tip portion 58 ofthe first helical spike 50 penetrates into the upper surface 19 of thevertebrae 14, while the tip portion 58 of the second helical spike 52projects through the lower surface 17 of the vertebrae 12. Continuedrotation of the interbody stabilizer 20 causes the tip portion 58 of thefirst helical spike 50 to cut a second helical segment 84 of the firsttunnel 80 in the vertebrae 14. Similarly, the continued rotation causesthe tip portion 58 of the second helical spike 52 to cut a secondhelical segment 104 of the second tunnel 100 in the vertebrae 12.

After another 90° to 180° of rotation of the interbody stabilizer 20,the tip portions 58 of the helical spikes 50 and 52 penetrate back outof the vertebrae 14 and 12, respectively, and into the interbody space62. More specifically, the tip portion 58 of the first helical spike 50projects through the upper surface 19 of the vertebrae 14 and the tipportion 58 of the second helical spike 52 projects through the lowersurface 17 of the vertebrae 12.

As the interbody stabilizer 20 is rotated further, t he tip portions 58of the helical spikes 50 and 52 move through the interbody space 62 andre-engage the vertebrae 12 and 14, respectively. The tip portion 58 ofthe firs t helical spike 50 penetrates the lower surface 17 of thevertebrae 12 and cuts a third helical segment 86 of the first tunnel 80in the vertebrae 12. Simultaneously, the tip portion 58 of the secondhelical spike 52 penetrates the upper surface 19 of the vertebrae 14 andcuts a third helical segment 106 of the second tunnel 100 in thevertebrae 14.

After further rotation of the interbody stabilizer 20, the tip portions58 of the helical spikes 50 and 52 a gain penetrate back out of thevertebrae 12 and 14, respectively and into the interbody space 62. Thetip portion 58 of the first helical spike 50 projects through the lowersurface 17 of the vertebrae 12, while the tip portion 58 of the secondhelical spike 52 projects through the upper surface 19 of the vertebrae14. The interbody stabilizer 20 is then rotated so that the tip portions58 of the helical spikes 50 and 52 move through the interbody space 62and re-engage the vertebrae 14 and 12, respectively. The tip portion 58of the first helical spike 50 again penetrates into the upper surface 19of the vertebrae 14, causing the tip portion 58 of the first helicalspike 50 to cut a fourth helical segment 88 of the first tunnel 80 inthe vertebrae 14. Similarly, the tip portion 58 of the second helicalspike 52 again penetrates through the lower surface 17 of the vertebrae12, causing the tip portion 58 of the second helical spike 52 to cut afourth helical segment 108 of the second tunnel 100 in the vertebrae 12.

This pattern of screwing the helical spikes 50 and 52 of the interbodystabilizer 20 into and out of each of the vertebrae 12 and 14 in analternating manner continues with each revolution of the platform 24 bythe driver. The continued rotation of the platform 24 embeds the helicalspikes 50 and 52 of the interbody stabilizer 20 into the vertebrae 12and 14 and attaches the interbody stabilizer to each of the vertebrae.With each rotation of the interbody stabilizer 20, the connectionbetween the interbody stabilizer and each of the vertebrae 12 and 14gets stronger. The attachment of the interbody stabilizer 20 to each ofthe vertebrae 12 and 14 thus fastens, or pins, the vertebrae together,yet spaced apart. Rotation of the platform 24 is terminated when the endsurface 38 of the platform seats against one or both of the sidesurfaces 16 and 18 of the vertebrae 12 and 14, respectively.

Once the interbody stabilizer 20 is implanted, bone graft material 130(shown schematically in FIGS. 1 and 2) for permanently fusing thevertebrae 12 and 14 is placed into the interbody space 62. Morespecifically, the bone graft material 130 is placed into a cavity 140defined by the helical spikes 50 and 52, the lower surface 17 of thevertebrae 12, and the upper surface 19 of the vertebrae 14. The bonegraft material 130, which may comprise bone chips and/or synthetic bonematerial, is placed into the cavity 140 through the axial passage 40 inthe platform 24 of the interbody stabilizer 20. A sufficient amount ofthe bone graft material 130 is placed into the cavity 140 to fill notonly the cavity, but also the entire interbody space 62.

When implanted, the interbody stabilizer 20 is attached to both of thevertebrae 12 and 14 and securely fastens the vertebrae together. Becauseeach of the helical spikes 50 and 52 penetrates into and subsequentlyout of each of the vertebrae 12 and 14, the helical spikes providemultiple fixation locations between the interbody stabilizer 20 and thevertebrae that pin the vertebrae together. The interbody stabilizer 20is therefore able to resist relative movement of the vertebrae 12 and 14toward or away from each other, and does not rely on surroundingligaments to stabilize the vertebrae. More specifically, the interbodystabilizer 20 resists relative movement of the vertebrae 12 and 14,through bending or rotation, along any one of the three planes of motion(sagittal, coronal, or horizontal). Thus, the interbody stabilizer 20 isable to maintain proper intervertebral spacing and provide effectivetemporary stabilization of the adjacent vertebrae 12 and 14, despitesubstantial forces on the interbody stabilizer caused by human bodymovement and muscle memory, while the bone graft material 130 fuses thevertebrae together. Advantageously, the interbody stabilizer 20 has asimple one-piece construct and does not require substantial cutting ofcortical bone (i.e., a reaming or tapping procedure) to prepare thevertebrae 12 and 14 to accept the interbody stabilizer. Thus, theinterbody stabilizer 20 is not only a simplified construct, but alsosimplifies the steps required for implantation into adjacent vertebrae.

FIGS. 6 and 7 illustrate an apparatus 210 constructed in accordance witha second embodiment of the present invention. In the second embodimentof FIGS. 6 and 7, reference numbers that are the same as those used inthe first embodiment of FIGS. 1-4 designate parts that are the same asparts in the first embodiment.

According to the second embodiment, the apparatus 210 comprises aninterbody stabilizer 220 having a platform 224. The platform 224includes a generally rectangular slot 232 that extends axially from afirst end 228 toward a second end 230 of the platform. Adjacent thefirst end 228, the platform 224 includes first and second segments ofexternal threads 234 and 236 that are separated by the slot 232. Theslot 232 and the threads 234 and 236 provide structure for connectingspinal fixation instrumentation to the platform 224. The first andsecond helical spikes 50 and 52 project from the end surface 38 at thesecond end 230 of the platform 224.

FIG. 6 illustrates how the interbody stabilizer 220 may be used forsegmental spinal fixation. Lumbar vertebrae L3 and L4, indicated byreference numbers 290 and 292, respectively, are shown in FIG. 6. Theinterbody stabilizer 220 according to the second embodiment of thepresent invention is implanted in the interbody space between thevertebrae 290 and 292. The interbody stabilizer 220 is implanted intothe vertebrae 290 and 2 92 in much the same manner a s described aboveregarding the first embodiment. A rotatable driver 270 (FIG. 7) fitsinto the slot 232 in the interbody stabilizer 220 and is used to rotatethe interbody stabilizer.

Once the interbody stabilizer 220 is implanted, spinal fixationinstrumentation such as a beam 280 which has been bent into a desiredshape by the surgeon, is placed into the slot 232 in the interbodystabilizer. A nut 282 is then screwed onto the threads 234 and 236 onthe platform 224 and tightened to secure the beam 280 to the interbodystabilizer 220. As in the first embodiment, the interbody stabilizer 220fastens the vertebrae 290 and 292 together and stabilizes the vertebraeuntil the bone graft material 130 placed in the cavity 140 definedinside each of the interbody stabilizers fuses the vertebrae. The beam280 helps to further support the vertebrae 290 and 292 until thevertebrae fuse together.

FIG. 8 illustrates an apparatus 310 constructed in accordance with athird embodiment of the present invention. In the third embodiment ofFIG. 8, reference numbers that are the same as those used in the firstembodiment of FIGS. 1-4 designate parts that are the same as parts inthe first embodiment.

According to the third embodiment, the interbody stabilizer 20 isimplanted into two cervical vertebrae 312 and 314 in the same manner asdescribed above regarding the first embodiment. The end surface 38 ofthe interbody stabilizer 20 seats against anterior surfaces 316 and 318of the vertebrae 312 and 314, respectively. As in the first embodiment,the interbody stabilizer 20 fastens the vertebrae 312 and 314 andstabilizes the vertebrae until the bone graft material 130 placed in thecavity 140 in the interbody stabilizer fuses the vertebrae.

FIGS. 9 and 10 illustrate an apparatus 410 constructed in accordancewith a fourth embodiment of the present invention. In the fourthembodiment of FIGS. 9 and 10, reference numbers that are the same asthose used in the first embodiment of FIGS. 1-4 designate parts that arethe same as parts in the first embodiment.

According to the fourth embodiment, the apparatus 410 comprises aninterbody stabilizer 420 having three helical spikes 430, 431, and 432projecting tangentially from the end surface 38 of the platform 24. Thespikes 430-432 are centered about the axis 22. As shown in FIG. 10, theconnecting portions 54 at the proximal ends 60 of the helical spikes430-432 are spaced 120° apart about the axis 22, which balances theinterbody stabilizer 420 and evenly distributes loads on the helicalspikes. As in the first embodiment of FIGS. 1-4, in the fourthembodiment of FIGS. 9 and 10, the cross-sectional diameter of theconnection portions 54 of the helical spikes 430-432 is greater than orequal to the cross-sectional diameter of the intermediate portions 56and the tip portions 58 of the helical spikes.

Each of the three helical spikes 430-432 extend in a helical patternabout the axis 22 at the same, constant radius R1. It is contemplated,however, that one or more of the helical spikes 430-432 could extendabout the axis 22 at different radiuses. Further, it is contemplatedthat the radius of one or more helical spikes 430-432 could increase ordecrease as the helical spikes extend away from the platform 24.

As shown in FIG. 9, the three helical spikes 430-432 have the same axiallength and also have the same circular cross-sectional shape. It iscontemplated, however, that one or more of the helical spikes 430-432could have different axial lengths. Further, it is contemplated that oneor more of the helical spikes 430-432 could have a differentcross-sectional shape, such as an oval shape. It also contemplated thatthe one or more of the helical spikes 430-432 could have differentcross-sectional shapes and/or areas (i.e., one spike being thicker orthinner than the other two spikes) . Finally, it is contemplated thatthe helical spikes 430-432 should have the same pitch, and that thepitch of the helical spikes would be selected based on the specificsurgical application and quality of the bone in which the interbodystabilizer 20 is to be implanted.

The tip portion 58 of each of the helical spikes 430-432 illustrated inFIG. 9 has an elongated conical shape for penetrating into a vertebraeas the platform 24 of the interbody stabilizer 420 is rotated in theclockwise direction. It should be understood that the tip portions 58 ofthe helical spikes 430-432 of the interbody stabilizer 420 couldalternatively be configured like the tip portions illustrated in FIG. 5.

The interbody stabilizer 420 according to the fourth embodiment of FIGS.9 and 10 is implanted into an adjacent pair of vertebrae in the samemanner as the interbody stabilizer 20 according to the first embodiment.Further, the interbody stabilizer 420 according to the fourth embodimentmay also be used to mount spinal fixation instrumentation as shown inthe second embodiment of FIGS. 6 and 7. When implanted, the interbodystabilizer 420 is attached to both of the adjacent vertebrae and fastensthe vertebrae together. Further, the interbody stabilizer 420 maintainsproper intervertebral spacing and provides effective temporarystabilization of the adjacent vertebrae while the bone graft materialplaced in the cavity in the interbody stabilizer fuses the vertebraetogether. Advantageously, the interbody stabilizer 420 is a simpleone-piece construct does not require substantial cutting of corticalbone (i.e., a reaming or tapping procedure) to prepare the adjacentvertebrae to accept the interbody stabilizer.

It should be noted that the interbody stabilizers according to thepresent invention can be used not only to stabilize a degenerative disc,but can also be used to correct spinal deformity such as scoliosis,kyphosis, lordosis, and spondylosisthesis.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. It should beunderstood that the method and apparatus according to the presentinvention could be used to attach and stabilize other adjacent bones,not just bones in the spine or pelvis. Further, it is contemplated thatthe present invention could comprise a single helical spike, or morethan three spikes. Such improvements, changes and modifications withinthe skill of the art are intended to be covered by the appended claims.

Having described the invention, I claim:
 1. An apparatus for implantation into an adjacent pair of vertebral bodies having first and second surfaces, respectively, that oppose each other, said apparatus, when implanted, being attached to each of the vertebral bodies and stabilizing the vertebral bodies while the vertebral bodies fuse together, said apparatus comprising: a platform having a third surface extending transverse to the first and second surfaces and transverse to a longitudinal axis of said apparatus; and at least two helical spikes shaped like intertwined corkscrews for embedding into each of the adjacent pair of vertebral bodies upon rotation of said platform to attach said at least two helical spikes to each of the vertebral bodies and thus fasten the vertebral bodies together, said at least two helical spikes projecting from said third surface of said platform and extending around said longitudinal axis, each of said at least two helical spikes having a connecting portion at a proximal end connected to said platform, said at least two helical spikes further having a tip portion at a distal end for penetrating the first and second surfaces and for screwing into the adjacent pair of vertebral bodies as said platform is rotated; said at least two helical spikes at least partially defining an internal cavity for receiving material that promotes fusion of the vertebral bodies.
 2. The apparatus of claim 1 wherein said platform includes an axially extending passage through which the material is placed into said internal cavity following implantation of said apparatus in the vertebral bodies.
 3. The apparatus of claim 1 wherein said at least two helical spikes comprises a pair of helical spikes, said proximal ends of said pair of helical spikes being spaced 180° apart.
 4. The apparatus of claim 1 wherein said at least two helical spikes comprises three helical spikes, said proximal ends of said three helical spikes being spaced 120° apart.
 5. The apparatus of claim 1 wherein said platform includes structure for connecting spinal fixation instrumentation.
 6. The apparatus of claim 1 wherein each of said at least two helical spikes has an intermediate portion extending between said connecting portion and said tip portion.
 7. The apparatus of claim 6 wherein said intermediate portion of each of said at least two helical spikes has a first cross-sectional diameter and said connecting portion of each of said at least two helical spikes has a second cross-sectional diameter that is greater than said first cross-sectional diameter.
 8. The apparatus of claim 6 wherein said intermediate portion of each of said at least two helical spikes has a first cross-sectional diameter and said connecting portion of each of said at least two helical spikes has a second cross-sectional diameter that is equal to said first cross-sectional diameter.
 9. The apparatus of claim 1 wherein said platform and said at least two helical spikes are made of a biocompatible material.
 10. The apparatus of claim 1 wherein said tip portion of each of said at least two helical spikes has a self-penetrating terminal end for penetrating into the bone as said platform is rotated.
 11. A method for attaching and stabilizing an adjacent pair of vertebral bodies while the vertebral bodies fuse together, the pair of vertebral bodies having first and second surfaces, respectively, that oppose each other, said method comprising the steps of: removing disc material disposed between the vertebral bodies to create an interbody space; providing an interbody stabilizer for insertion into the interbody space by implanting the interbody stabilizer into both of the adjacent pair of vertebral bodies, the interbody stabilizer comprising a platform and at least two helical spikes that are shaped like intertwined corkscrews, the platform having a third surface extending transverse to the first and second surfaces of the vertebral bodies, the at least two helical spikes projecting from the third surface of the platform and extending around a common longitudinal axis, the at least two helical spikes at least partially defining an internal cavity for receiving material that promotes fusion of the vertebral bodies; embedding each of the at least two helical spikes of the interbody stabilizer into the adjacent pair of vertebral bodies by rotating the platform of the interbody stabilizer which causes each of the at least two helical spikes to penetrate into and subsequently out of each of the vertebral bodies in an alternating manner to attach the interbody stabilizer to each of the vertebral bodies and thus fasten the vertebral bodies together; and placing material that promotes fusion of the vertebral bodies into the internal cavity in the interbody stabilizer.
 12. The method of claim 11 further comprising the step of attaching spinal fixation instrumentation to the platform of the interbody stabilizer. 